| Abstract: | The present paper investigates the temperature structure of the X-ray
emitting plasma in the core of the Perseus cluster using the 1.8--20.0 keV data
obtained with the Soft X-ray Spectrometer (SXS) onboard the Hitomi Observatory.
A series of four observations were carried out, with a total effective exposure
time of 338 ks and covering a central region $sim7’$ in diameter. The SXS was
operated with an energy resolution of $sim$5 eV (full width at half maximum)
at 5.9 keV. Not only fine structures of K-shell lines in He-like ions but also
transitions from higher principal quantum numbers are clearly resolved from Si
through Fe. This enables us to perform temperature diagnostics using the line
ratios of Si, S, Ar, Ca, and Fe, and to provide the first direct measurement of
the excitation temperature and ionization temperature in the Perseus cluster.
The observed spectrum is roughly reproduced by a single temperature thermal
plasma model in collisional ionization equilibrium, but detailed line ratio
diagnostics reveal slight deviations from this approximation. In particular,
the data exhibit an apparent trend of increasing ionization temperature with
increasing atomic mass, as well as small differences between the ionization and
excitation temperatures for Fe, the only element for which both temperatures
can be measured. The best-fit two-temperature models suggest a combination of 3
and 5 keV gas, which is consistent with the idea that the observed small
deviations from a single temperature approximation are due to the effects of
projection of the known radial temperature gradient in the cluster core along
the line of sight. Comparison with the Chandra/ACIS and the XMM-Newton/RGS
results on the other hand suggests that additional lower-temperature components
are present in the ICM but not detectable by Hitomi SXS given its 1.8--20 keV
energy band. |
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